Operating under contract with the
United States Atomic Energy Commission

A BRIEF HISTORY OF
ELEMENT DISCOVERY, SYNTHESIS,
AND ANALYSIS

It is well known that the number of elements has grown from
four in the days of the Greeks to 103 at present, but the change in
methods needed for their discovery is not so well known. Up until
1939, only 88 naturally occurring elements had been discovered.
It took a dramatic modern technique (based on Ernest O. Lawrence's
Nobel-prize-winning atom smasher, the cyclotron) to synthesize
the most recently discovered elements. Most of these
recent discoveries are directly attributed to scientists working under
the Atomic Energy Commission at the University of California's
Radiation Laboratory at Berkeley.

But it is apparent that our present knowledge of the elements
stretches back into history: back to England's Ernest Rutherford,
who in 1919 proved that, occasionally, when an alpha particle
from radium strikes a nitrogen atom, either a proton or a hydrogen
nucleus is ejected; to the Dane Niels Bohr and his 1913 idea
of electron orbits; to a once unknown Swiss patent clerk, Albert
Einstein, and his now famous theories; to Poland's Marie Curie
who, in 1898, with her French husband Pierre laboriously isolated
polonium and radium; back to the French scientist H. A. Becquerel,
who first discovered something he called a "spontaneous
emission of penetrating rays from certain salts of uranium"; to
the German physicist W. K. Roentgen and his discovery of x rays
in 1895; and back still further.

During this passage of scientific history, the very idea of
"element" has undergone several great changes.

The early Greeks suggested earth, air, fire, and water as being
the essential material from which all others were made. Aristotle
considered these as being combinations of four properties: hot,
cold, dry, and moist (see Fig. 1).

Fig. 1. The elements as proposed by the early Greeks.

Later, a fifth "essence," ether, the building material of the
heavenly bodies was added.

Paracelsus (1493-1541) introduced the three alchemical symbols
salt, sulfur, and mercury. Sulfur was the principle of combustability,
salt the fixed part left after burning (calcination), and
mercury the essential part of all metals. For example, gold and
silver were supposedly different combinations of sulfur and
mercury.

Robert Boyle in his "Sceptical Chymist" (1661) first defined
the word element in the sense which it retained until the discovery
of radioactivity (1896), namely, a form of matter that could not be
split into simpler forms.

The first discovery of a true element in historical time was
that of phosphorus by Dr. Brand of Hamburg, in 1669. Brand
kept his process secret, but, as in modern times, knowledge of
the element's existence was sufficient to let others, like Kunkel
and Boyle in England, succeed independently in isolating it
shortly afterward.

As in our atomic age, a delicate balance was made between
the "light-giving" (desirable) and "heat-giving" (feared) powers
of a discovery. An early experimenter was at first "delighted with
the white, waxy substance that glowed so charmingly in the dark
of his laboratory," but later wrote, "I am not making it any
more for much harm may come of it."

Robert Boyle wrote in 1680 of phosphorus, "It shone so
briskly and lookt so oddly that the sight was extreamly pleasing,
having in it a mixture of strangeness, beauty and frightfulness."

These words describe almost exactly the impressions of eye
witnesses of the first atom bomb test at Alamagordo, New Mexico,
July 16, 1945.

For the next two and three-quarters centuries the chemists
had much fun and some fame discovering new elements. Frequently
there was a long interval between discovery and recognition.
Thus Scheele made chlorine in 1774 by the action of "black
manganese" (manganese dioxide) on concentrated muriatic acid
(hydrochloric acid), but it was not recognized as an element till
the work of Davy in 1810.

Occasionally the development of a new technique would lead
to the "easy" discovery of a whole group of new elements. Thus
Davy, starting in 1807, applied the method of electrolysis, using
a development of Volta's pile as a source of current; in a short
time he discovered aluminum, barium, boron, calcium, magnesium,
potassium, sodium, and strontium.

The invention of the spectroscope by Bunsen and Kirchhoff
in 1859 provided a new tool which could establish the purity of
substances already known and lead to the discovery of others.
Thus, helium was discovered in the sun's spectrum by Jansen and
isolated from uranite by Ramsay in 1895.

The discovery of radioactivity by Becquerel in 1896 (touched
off by Roentgen's discovery of x rays the year before) gave an
even more sensitive method of detecting the presence or absence
of certain kinds of matter. It is well known that Pierre and Marie
Curie used this new-found radioactivity to identify the new
elements polonium and radium. Compounds of these new elements
were obtained by patient fractional recrystallization of their salts.

The "explanation" of radioactivity led to the discovery of
isotopes by Rutherford and Soddy in 1914, and with this discovery
a revision of our idea of elements became necessary. Since Boyle,
it had been assumed that all atoms of the individual elements
were identical and unlike any others, and could not be changed
into anything simpler. Now it became evident that the atoms of
radioactive elements were constantly changing into other elements,
thereby releasing very large amounts of energy, and that
many different forms of the same element (lead was the first
studied) were possible. We now think of an element as a form of
matter in which all atoms have the same nuclear charge.

The human mind has always sought order and simplification
of the external world; in chemistry the fruitful classifications
were Dobereiner's Triads (1829), Newland's law of octaves (1865),
and Mendeleev's periodic law (1869). The chart expressing this
periodic law seemed to indicate the maximum extent of the elements
and gave good hints "where to look for" and "the probable
properties of" the remaining ones (see Fig. 2).

By 1925, all but four of the slots in the 92-place file had been
filled. The vacancies were at 43, 61, 85, and 87.

Workers using traditional analytical techniques continued to
search for these elements, but their efforts were foredoomed to
failure. None of the nuclei of the isotopes of elements 43, 61, 85,[Pg 6]
and 87 are stable; hence weighable quantities of them do not exist
in nature, and new techniques had to be developed before we
could really say we had "discovered" them.

In 1919, Rutherford accomplished scientifically what medieval
alchemists had failed to do with "magic" experiments and
other less sophisticated techniques. It wasn't gold (the goal of
the alchemists) he found but something more valuable with even
greater potential for good and evil: a method of transmuting one
element into another. By bombarding nitrogen nuclei with alpha
particles from radium, he found that nitrogen was changed into
oxygen.

The process for radioactive transmutation is somewhat like
a common chemical reaction. An alpha particle, which has the
same charge (+2) and atomic mass (4) as a helium nucleus, penetrates
the repulsive forces of the nitrogen nucleus and deposits
one proton and one neutron; this changes the nitrogen atom into
an oxygen atom. The reaction is written

The number at the lower left of each element symbol in the
above reaction is the proton number. This number determines
the basic chemical identity of an atom, and it is this number
scientists must change before one element can be transformed into
another. The common way to accomplish this artificially is by
bombarding nuclei with nuclear projectiles.

Rutherford used naturally occurring alpha particles from
radium as his projectiles because they were the most effective he
could then find. But these natural alpha particles have several
drawbacks: they are positively charged, like the nucleus itself,
and are therefore more or less repulsed depending on the proton
number of the element being bombarded; they do not move fast
enough to penetrate the nuclei of heavier elements (those with
many protons); and, for various other reasons (some of them
unexplained), are inefficient in breaking up the nucleus. It is
estimated that only 1 out of 300,000 of these alpha particles will
react with nitrogen.

Physicists immediately began the search for artificial means
to accelerate a wider variety of nuclear particles to high energies.

Protons, because they have a +1 charge rather than the +2
charge of the alpha particles, are repulsed less strongly by the
positive charge on the nucleus, and are therefore more useful as
bombarding projectiles. In 1929, E. T. S. Walton and J. D. Cockcroft
passed an electric discharge through hydrogen gas, thereby
removing electrons from the hydrogen atom; this left a beam
of protons (i. e., hydrogen ions), which was then accelerated by
high voltages. This Cockcroft-Walton voltage multiplier accelerated
the protons to fairly high energies (about 800,000 electron
volts), but the protons still had a plus charge and their energies
were still not high enough to overcome the repulsive forces
(Coulombic repulsion) of the heavier nuclei.

A later development, the Van de Graaff electrostatic generator,
produced a beam of hydrogen ions and other positively
charged ions, and electrons at even higher energies. An early
model of the linear accelerator also gave a beam of heavy positive
ions at high energies. These were the next two instruments devised
in the search for efficient bombarding projectiles. However, the
impasse continued: neither instrument allowed scientists to crack
the nuclei of the heavier elements.

Ernest O. Lawrence's cyclotron, built in 1931, was the first
device capable of accelerating positive ions to the very high
energies needed. Its basic principle of operation is not difficult
to understand. A charged particle accelerated in a cyclotron is
analogous to a ball being whirled on a string fastened to the top of
a pole. A negative electric field attracts the positively charged
particle (ball) towards it and then switches off until the particle
swings halfway around; the field then becomes negative in front
of the particle again, and again attracts it. As the particle moves
faster and faster it spirals outward in an ever increasing circle,
something like a tether ball unwinding from a pole. The energies
achieved would have seemed fantastic to earlier scientists. The
Bevatron, a modern offspring of the first cyclotron, accelerates
protons to 99.13% the speed of light, thereby giving them 6.2
billion electron volts (BeV).

Another instrument, the heavy-ion linear accelerator (Hilac),
accelerates ions as heavy as neon to about 15% the speed of
light. It is called a linear accelerator because it accelerates particles
in a straight line. Stanford University is currently (1963) in
the process of building a linear accelerator approximately two
miles long which will accelerate charged particles to 99.9% the
speed of light.

But highly accelerated charged particles did not solve all of
science's questions about the inner workings of the nucleus.

In 1932, during the early search for more efficient ways to
bombard nuclei, James Chadwick discovered the neutron. This
particle, which is neutral in charge and is approximately the same
mass as a proton, has the remarkable quality of efficiently producing
nuclear reactions even at very low energies. No one exactly
knowns why. At low energies, protons, alpha particles, or other
charged particles do not interact with nuclei because they cannot
penetrate the electrostatic energy barriers. For example, slow
positive particles pick up electrons, become neutral, and lose
their ability to cause nuclear transformations. Slow neutrons, on
the other hand, can enter nearly all atomic nuclei and induce
fission of certain of the heavier ones. It is, in fact, these properties
of the neutron which have made possible the utilization of atomic
energy.

With these tools, researchers were not long in accurately
identifying the missing elements 43, 61, 85, and 87 and more—indeed,
the list of new elements, isotopes, and particles now
seems endless.

Element 43 was "made" for the first time as a result of
bombarding molybdenum with deuterons in the Berkeley cyclotron.
The chemical work of identifying the element was done by
Emilio Segrè and others then working at Palermo, Sicily, and
they chose to call it technetium, because it was the element first
made by artificial technical methods.

Element 61 was made for the first time from the fission disintegration
products of uranium in the Clinton (Oak Ridge)[Pg 9]
reactor. Marinsky and Glendenin, who did the chemical work of
identification, chose to call it promethium because they wished to
point out that just as Prometheus stole fire (a great force for good
or evil) from the hidden storehouse of the gods and presented it to
man, so their newly assembled reactor delivered to mankind an
even greater force, nuclear energy.

Element 85 is called astatine, from the Greek astatos, meaning
"unstable," because astatine is unstable (of course all other
elements having a nuclear charge number greater than 84 are
unstable, too). Astatine was first made at Berkeley by bombarding
bismuth with alpha particles, which produced astatine and released
two neutrons. The element has since been found in nature as a
small constituent of the natural decay of actinium.

The last of the original 92 elements to be discovered was
element 87, francium. It was identified in 1939 by French scientist
Marguerite Perey.

Children have a game in which they pile blocks up to see
how high they can go before they topple over. In medieval times,
petty rulers in their Italian states vied with one another to see who
could build the tallest tower. Some beautiful results of this game
still remain in Florence, Siena, and other Italian hill cities. Currently,
Americans vie in a similar way with the wheelbase and
overall length of their cars. After 1934, the game among scientists
took the form of seeing who could extend the length of the
periodic system of the elements; as with medieval towers, it was
Italy that again began with the most enthusiasm and activity
under the leadership of Enrico Fermi.

Merely adding neutrons would not be enough; that would
make only a heavier isotope of the already known heaviest elements,
uranium. However, if the incoming neutron caused some
rearrangement within the nucleus and if it were accompanied by
expulsion of electrons, that would make a new element. Trials by
Fermi and his co-workers with various elements led to unmistakeable
evidence of the expulsion of electrons (beta activity) with at
least four different rates of decay (half-lives). Claims were advanced[Pg 10]
for the creation of elements 93 and 94 and possibly further (the
transuranium elements, Table I). Much difficulty was experienced,
however, in proving that the activity really was due to the formation
of elements 93 and 94. As more people became interested
and extended the scope of the experiments, the picture became
more confused rather than clarified. Careful studies soon showed
that the activities did not decay logarithmically—which means
that they were caused by mixtures, not individual pure substances—and
the original four activities reported by Fermi grew
to at least nine.

As a matter of fact, the way out of the difficulty had been
indicated soon after Fermi's original announcement. Dr. Ida
Noddack pointed out that no one had searched among the products
of Fermi's experiment for elements lighter than lead, but no one
paid any attention to her suggestion at the time. The matter was
finally cleared up by Dr. Otto Hahn and F. Strassmann. They
were able to show that instead of uranium having small pieces like
helium nuclei, fast electrons, and super-hard x-rays, knocked off
as expected, the atom had split into two roughly equal pieces,
together with some excess neutrons. This process is called nuclear
fission. The two large pieces were unstable and decayed further
with the loss of electrons, hence the β activity. This process is so
complicated that there are not, as originally reported, only four
half-lives, but at least 200 different varieties of at least 35 different
elements. The discovery of fission attended by the release of
enormous amounts of energy led to feverish activity on the part
of physicists and chemists everywhere in the world. In June 1940,
McMillan and Abelson presented definite proof that element 93
had been found in uranium penetrated by neutrons during deuteron
bombardment in the cyclotron at the University of California
Radiation Laboratory.

The California scientists called the newly discovered element
neptunium, because it lies beyond the element uranium just as
the planet Neptune lies beyond Uranus. The particular isotope
formed in those first experiments was 93Np239; this is read neptunium
having a nuclear charge of 93 and an atomic mass number[Pg 11]
of 239. It has a half-life of 2.3 days, during which it gives up
another electron (β particle) and becomes element 94, or plutonium
(so called after Pluto, the next planet beyond Neptune). This
particular form of plutonium (94Pu239) has such a long half-life
(24,000 years) that it could not be detected. The first isotope of
element 94 to be discovered was Pu238, made by direct deuteron
bombardment in the Berkeley 60-inch cyclotron by Radiation
Laboratory scientists Seaborg, McMillan, Kennedy, and Wahl; it
had an α-decay half-life of 86.4 years, which gave it sufficient
radioactivity so that its chemistry could be studied.

Having found these chemical properties in Pu238, experimenters
knew 94Pu239 would behave similarly. It was soon shown
that the nucleus of 94Pu239 would undergo fission in the same way
as 92U235 when bombarded with slow neutrons and that it could
be produced in the newly assembled atomic pile. Researchers
wished to learn as much as possible about its chemistry; therefore,
during the summer of 1942 two large cyclotrons at St. Louis and
Berkeley bombarded hundreds of pounds of uranium almost continuously.
This resulted in the formation of 200 micrograms of
plutonium. From this small amount, enough of the chemical
properties of the element were learned to permit correct design of
the huge plutonium-recovery plant at Hanford, Washington. In
the course of these investigations, balances that would weigh up
to 10.5 mg with a sensitivity of 0.02 microgram were developed.
The "test tubes" and "beakers" used had internal diameters of
0.1 to 1 mm and could measure volumes of 1/10 to 1/10,000 ml
with an accuracy of 1%. The fact that there was no intermediate
stage of experimentation, but a direct scale-up at Hanford of ten
billion times, required truly heroic skill and courage.

By 1944 sufficient plutonium was available from uranium
piles (reactors) so that it was available as target material for
cyclotrons. At Berkeley it was bombarded with 32-MeV doubly
charged helium ions, and the following reactions took place:

This is to be read: plutonium having an atomic number of 94 (94
positively charged protons in the nucleus) and a mass number of
239 (the whole atom weighs approximately 239 times as much as
a proton), when bombarded with alpha particles (positively
charged helium nuclei) reacts to give off a neutron and a new element,
curium, that has atomic number 96 and mass number 242.
This gives off alpha particles at such a rate that half of it has
decomposed in 150 days, leaving plutonium with atomic number 94
and mass number 238. The radiochemical work leading to the
isolation and identification of the atoms of element 96 was done
at the metallurgical laboratory of the University of Chicago.

The intense neutron flux available in modern reactors led to
a new element, americium (Am), as follows:

The notation (n, γ) means that the plutonium absorbs a neutron
and gives off some energy in the form of gamma rays (very hard x
rays); it first forms 94Pu240 and then 94Pu241, which is unstable and
gives off fast electrons (β), leaving 95Am241.

Berkelium and californium, elements 97 and 98, were produced
at the University of California by methods analogous to
that used for curium, as shown in the following equations:

and

The next two elements, einsteinium (99Es) and fermium
(100Fm), were originally found in the debris from the thermonuclear
device "Mike," which was detonated on Eniwetok atoll
November 1952. (This method of creating new substances is somewhat
more extravagant than the mythical Chinese method of
burning down a building to get a roast pig.)

These elements have since been made in nuclear reactors and
by bombardment. This time the "bullet" was N14 stripped of
electrons till it had a charge of +6, and the target was plutonium.

Researchers at the University of California used new techniques
in forming and identifying element 101, mendelevium. A
very thin layer of 99Es253 was electroplated onto a thin gold foil
and was then bombarded, from behind the layer, with 41-MeV α
particles. Unchanged 99Es253 stayed on the gold, but those atoms
hit by α particles were knocked off and deposited on a "catcher"
gold foil, which was then dissolved and analyzed (Fig. 3). This
freed the new element from most of the very reactive parent substances,
so that analysis was easier. Even so, the radioactivity
was so weak that the new element was identified "one atom at a
time"; this is possible because its daughter element, fermium,
spontaneously fissions and releases energy in greater bursts than
any possible contaminant.

Fig. 3. The production of mendelevium.

In 1957, in Stockholm, element 102 was reported found by
an international team of scientists (who called it nobelium), but
diligent and extensive research failed to duplicate the Stockholm
findings. However, a still newer technique developed at Berkeley
showed the footprints—if not the living presence—of 102 (see
Fig. 4). The rare isotope curium-246 is coated on a small piece of
nickel foil, enclosed in a helium-filled container, and placed in the[Pg 16]
heavy-ion linear accelerator (Hilac) beam. Positively charged atoms
of element 102 are knocked off the foil by the beam, which is of
carbon-12 or carbon-13 nuclei, and are deposited on a negatively
charged conveyor apron. But element 102 doesn't live long enough
to be actually measured. As it decays, its daughter product,
100Fm250, is attracted onto a charged aluminum foil where it can be
analyzed. The researchers have decided that the hen really did
come first: they have the egg; therefore the hen must have existed.
By measuring the time distance between target and daughter
product, they figure that the hen-mother (element 102) must have
a half-life of three seconds.

Fig. 4. The experimental arrangement
used in the discovery of element 102.

In an experiment completed in 1961, researchers at the
University of California at Berkeley unearthed similar "footprints"
belonging to element 103 (named lawrencium in honor of Nobel
prizewinner Ernest O. Lawrence). They found that the bombardment
of californium with boron ions released α particles which
had an energy of 8.6 MeV and decayed with a half-life of 8 ± 2
seconds. These particles can only be produced by element 103,
which, according to one scientific theory, is a type of "dinosaur"
of matter that died out a few weeks after creation of the universe.

The half-life of lawrencium (Lw) is about 8 seconds, and its
mass number is thought to be 257, although further research is
required to establish this conclusively.

Research on lawrencium is complicated. Its total α activity
amounts to barely a few counts per hour. And, since scientists
had the α-particle "footprints" only and not the beast itself, the
complications increased. Therefore no direct chemical techniques
could be used, and element 103 was the first to be discovered solely
by nuclear methods.[A]

For many years the periodic system was considered closed
at 92. It has now been extended by at least eleven places (Table I),
and one of the extensions (plutonium) has been made in truckload
lots. Its production and use affect the life of everyone in the
United States and most of the world.

Surely the end is again in sight, at least for ordinary matter,
although persistent scientists may shift their search to the other-world
"anti" particles. These, too, will call for very special
techniques for detection of their fleeting presence.

Early enthusiastic researchers complained that a man's life
was not long enough to let him do all the work he would like on
an element. The situation has now reached a state of equilibrium;
neither man nor element lives long enough to permit all the
desired work.

[A] In August 1964 Russian scientists claimed that
they created element 104 with a half-life of about
0.3 seconds by bombarding plutomium with accelerated
neon-22 ions.

Pu239; the fissionable isotope of plutonium, was also discovered in 1941 by J. W. Kennedy, G. T. Seaborg, E. Segrè and A. C. Wahl; University of California at Berkeley; slow-neutron bombardment of U238 in the 60-inch cyclotron.

[B] A 1957 claim for the synthesis and identification of element 102 was accepted at that time by
the International Union of Pure and Applied Chemistry, and the name nobelium (symbol No) was
adopted. The University of California scientists, A. Ghiorso et al., cited here believe they have
disproved the earlier claim and have the right to suggest a different name for the element.

Transcriber's Note

Table I (The Transuranium Elements) was originally located in the middle of the text on pages 12-13.
To improve readability of the e-book text, it has been relocated to the end of the text.